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Integrated Load Switches Versus Discrete Mosfets (Rev. A)

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Integrated Load Switches Versus Discrete Mosfets (Rev. A) Application Report SLVA716A–December 2015–Revised May 2019 Integrated Load Switches Versus Discrete MOSFETs Alek Kaknevicius and Arthur Huang ................................................................. Drivers and Load Switches ABSTRACT The most common approach to load switching solutions is to use a Power MOSFET surrounded by discrete resistors and capacitors; however, in most cases using a fully integrated load switch has significant advantages. While both discrete and integrated load switching solutions perform the same basic function (turn on and turn off), distinctions exist, such as the transient behavior and total solution size. This application report highlights many drawbacks and limitations of a discrete switching solution and discusses how they can be overcome with an integrated load switch. Contents 1 Summary of Load Switching................................................................................................ 2 2 PMOS Discrete Circuit #1 .................................................................................................. 3 2.1 Performance......................................................................................................... 3 2.2 Adding a Resistor to Slow the Output Rise Time............................................................... 5 2.3 Vth Voltage Disadvantage .......................................................................................... 6 2.4 Active Low Disadvantage .......................................................................................... 6 3 PMOS Discrete Circuit #2 .................................................................................................. 6 3.1 Performance......................................................................................................... 7 3.2 VIN Leakage Disadvantage ....................................................................................... 9 3.3 Adding a Resistor to Slow the Output Rise Time............................................................... 9 4 PMOS Discrete Circuit #3................................................................................................. 10 4.1 Performance ....................................................................................................... 11 4.2 Disadvantage when Applying VIN ............................................................................... 12 5 PMOS Discrete Circuit #4 ................................................................................................ 14 6 NMOS Discrete Circuits ................................................................................................... 15 7 Load Switches .............................................................................................................. 16 7.1 Performance ....................................................................................................... 17 7.2 Size Advantage.................................................................................................... 19 7.3 Feature Advantages .............................................................................................. 19 8 Conclusion .................................................................................................................. 21 9 References .................................................................................................................. 22 SLVA716A–December 2015–Revised May 2019 Integrated Load Switches Versus Discrete MOSFETs 1 Submit Documentation Feedback Copyright © 2015–2019, Texas Instruments Incorporated Summary of Load Switching www.ti.com 1 Summary of Load Switching A typical system involves a power supply and multiple loads which require various load currents. In most cases, the system must independently control which loads are on, when they are turned on, and how quickly they turn on. As previously mentioned, this power switching, as Figure 1 shows, can be implemented using a discrete MOSFET circuit or an integrated load switch. Power Switches Load Power Supply Load Load Figure 1. Power Switching A discrete MOSFET circuit contains several components to control the turnon and turnoff of a discrete power MOSFET. These circuits can be enabled or disabled by using a GPIO signal from a microcontroller. Figure 2 shows several of these circuits. VIN VOUT VIN VOUT VIN VOUT ON ON ON Figure 2. PMOS Discrete Circuits Load switches can also be used to open and close the connection between the power rail and the corresponding load. These integrated devices have several benefits and features while they are enabled, disabled, or even switching between the two states. Figure 3 shows a load switch circuit. Power Supply VIN VOUT Load ON EN GND OFF Load Switch Figure 3. Load Switch Circuit The following sections use switching waveforms to show and compare the discrete circuits and integrated load switches when used for power switching. 2 Integrated Load Switches Versus Discrete MOSFETs SLVA716A–December 2015–Revised May 2019 Submit Documentation Feedback Copyright © 2015–2019, Texas Instruments Incorporated www.ti.com PMOS Discrete Circuit #1 2 PMOS Discrete Circuit #1 The simplest discrete circuit that can be used for power switching is a PMOS transistor whose gate is driven by a GPIO. VIN VOUT ON Figure 4. PMOS Discrete Circuit #1 The benefit of this solution is simplicity. Only one component is required and the operation is simple—when the GPIO is high, the PMOS is turned off, and when the GPIO is pulled low, the PMOS is turned on. 2.1 Performance The performance of the PMOS solution is evaluated by looking at the way the PMOS transistor is able to switch on with a load attached. For the purpose of this application report, a resistive load of 1 Ω and capacitive load of 4.7 µF are used. Figure 5 shows this type of circuit. 3.3 V VIN VOUT 47 µF 4.7 µF 1 3.3 V ON Figure 5. PMOS Discrete Circuit #1 with a Resistive and Capacitive Load The input capacitance was chosen to be 10 times higher than the output capacitance to show a solution with strong compensation for transient currents seen during start up. Figure 6 shows the typical turn on behavior of this circuit. SLVA716A–December 2015–Revised May 2019 Integrated Load Switches Versus Discrete MOSFETs 3 Submit Documentation Feedback Copyright © 2015–2019, Texas Instruments Incorporated PMOS Discrete Circuit #1 www.ti.com Figure 6. Turn on Behavior for PMOS Discrete Circuit #1 When the GPIO signal (ON) is brought low, the PMOS transistor is turned on and its load is connected to the power supply (VIN). Because the PMOS has no controlled turn on, the voltage on the power supply decreases heavily due to the sudden demand for current. As the current ramps up to its final value, the voltage on VIN stabilizes. Figure 7 shows the stabilizing behavior of the PMOS Discrete circuit. The resistive load is removed and the capacitive load is left on the output when observing the magnitude of the inrush current. Figure 7. Inrush Current for PMOS Discrete Circuit #1 With only a small capacitance of 4.7 µF, the uncontrolled turnon manages to generate over 2 A of inrush current. As the output capacitance increases, the inrush current also increases at the same rate. Use Equation 1 to calculate inrush current. 4 Integrated Load Switches Versus Discrete MOSFETs SLVA716A–December 2015–Revised May 2019 Submit Documentation Feedback Copyright © 2015–2019, Texas Instruments Incorporated www.ti.com PMOS Discrete Circuit #1 dV IC= ´ INRUSH LOAD dt where • IINRUSH = amount of inrush current caused by a capacitance • CLOAD = total capacitance • dV = change in voltage during ramp up • dt = rise time (during voltage ramp up) (1) This current can lead to input voltage decreases, power supply failure, or PCB trace damage. If the input voltage at the load switch becomes lower than expected, such as in the case of the MOSFET startup, then other modules or subsystems on the same power rail may reset because of a low input voltage and which transitions the system into an undesired state. For more information on the negative effects of inrush current, see the Managing Inrush Current Application Note. 2.2 Adding a Resistor to Slow the Output Rise Time A resistor is added to the gate of the PMOS to help slow down the rise time of the output voltage (and reduce the inrush current). This additional resistor limits the amount of current that charges or discharges the gate and turns the PMOS off or on. 3.3 V VIN VOUT 47 µF 4.7 µF 1 20 k 3.3 V ON Figure 8. PMOS Discrete Circuit #1 with a 20-kΩ Resistor on the PMOS Gate Figure 9 shows the effect of a 20-kΩ resistor on the gate of the PMOS. Figure 9. Turnon Behavior of PMOS Discrete Circuit #1 with a 20-kΩ Resistor on the PMOS Gate SLVA716A–December 2015–Revised May 2019 Integrated Load Switches Versus Discrete MOSFETs 5 Submit Documentation Feedback Copyright © 2015–2019, Texas Instruments Incorporated PMOS Discrete Circuit #1 www.ti.com With the 20-kΩ resistor added, the rise time increases. The power supply is now able to manage the inrush current without any significant voltage drop. While this resistance may work for the 4.7-μF load, a higher output capacitance requires a higher rise time and therefore more resistance on the gate. While adding resistance to the gate of the PMOS helps in increasing the rise time of the output, it also increases the fall time when the PMOS is turned off. This can be a disadvantage for systems which need the output load discharged quickly for faster system operation. 2.3 Vth Voltage Disadvantage Aside from
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